1. Field of the Invention
The present invention broadly relates to improved electrical energy storage systems, more particularly to metal halogen battery systems. More specifically, the invention relates to a method of and apparatus for dynamically maintaining the hydrate formation temperature within a hydrate forming solution during charge.
2. Description of the Prior Art
The electrical energy storage systems of the type referred to herein, (e.g., a zinc-chlorine battery) utilize a halogen hydrate as a source of halogen component for reduction at a normally positive electrode, and an oxidizable metal adapted to become oxidized at a normally negative electrode during the normal discharge of the storage system. An aqueous electrolyte is employed for replenishing the supply of the halogen component as it becomes reduced at the positive electrode. The electrolyte contains the dissolved ions of the oxidized metal and the reduced halogen, and is circulated between the electrode area and a storage area containing halogen hydrate, which progressively decomposes during a normal discharge of the electrical energy system liberating additional elemental halogen to be consumed at the positive electrode. Electrical energy storage systems or battery systems of this type are described in prior patents owned by the same assignee as the present invention, such as U.S. Pat. No. 3,713,888, U.S. Pat. No. 3,993,502, U.S. Pat. No. 4,001,036, and U.S. Pat. No. 4,146,680. Such systems are also described in U.S. patents owned by the assignee of the present invention, such as U.S. Pat. No. 4,413,042, issued Nov. 1, 1983, entitled Inert Gas Rejection System for Metal Halogen Batteries, and U.S. Pat. No. 4,400,446, issued Aug. 23, 1983, and entitled Halogen Hydrate Storage Device for Mobile Zinc-Chloride Battery Systems, U.S. Pat. No. 4,415,847, issued Nov. 15, 1983, entitled Method and Apparatus for Supplying Cooling Liquid to a Storage Battery.
The basic operation of a metal halogen battery, such as an aqueous zinc-chloride battery system with graphite and/or other stable electrode substrates is as follows. In charge, an electrolyte pump delivers aqueous electrolyte to pockets between pairs of porous graphite-chlorine electrodes in a battery stack comprised of a plurality of cells. The electrolyte passes through the porous chlorine electrodes into a chamber between opposite polarity electrodes, flows up between the electrodes, then flows back into the battery sump. Chlorine gas liberated from porous graphite electrode substrates is pumped by a gas pump, and before entering the gas pump, the chlorine is mixed with electrolyte chilled by a chiller unit. The chlorine and chilled electrolyte are mixed in the gas pump, chlorine hydrate forms, and the chlorine hydrate-electrolyte mixture is deposited in the store. In discharge, chlorine is liberated from hydrate by decomposition of chlorine hydrate in the store by injection of warm electrolyte from the sump. On development of the required chlorine gas pressure in the store, the chlorine is injected and mixed with and dissolved in the electrolyte, which is then fed into the porous electrodes in the battery stack. The battery stack is then discharged, wherein the electrode dissolution of zinc occurs at the zinc electrode, reduction of the dissolved chlorine occurs at the chlorine electrode, power is available from the battery terminals, and zinc chloride formed in the electrolyte by reaction of zinc and chlorine to form zinc chloride.
During charge, in order for the hydrate to form properly, the temperature within the electrolyte must be closely controlled within tolerances on the order of a few tenths of a degree centigrade. A problem with presently available chiller units is that they have proven generally deficient in providing closely controlled temperatures. Thus it is an object of the present invention to provide a method and apparatus for controlling the hydrate formation tempterature during charge, and a further object to provide very precise temperature control of the hydrate forming electrolyte. Another object is to provide a method and apparatus for controlling the hydrate formation temperature during charge which utilizes servo control based on rate control, thereby allowing implementation using comparatively inexpensive bi-directional AC servo motors and the like.
These and other objects and advantages of the invention will become apparent from the subsequent description and the appended claims taken in conjunction with the accompanying drawings. It is also to be understood that the invention herein is applicable to numerous different constructional arrangements of metal halogen battery systems.